Large catadioptric objective

ABSTRACT

An objective system of high relative aperture for low-lightlevel application comprises a catadioptric system, utilizing a single glass type and only spherical surfaces, and includes three large correctors in front of the primary, and a field corrector element located near the focal plane. One of the large corrector elements is combined with the primary to form a Mangin mirror, the secondary is made coincident with the rear surface of the intermediate corrector, and the forwardmost of the corrector elements is positive.

United States Amon et al.

541 LARGE CATADIOPTRIC OBJECTIVE [75] Inventors: Max Amon, Farmingdale; Seymour Rosin, Massapequa, both of [73] Assignee: Kollsman Instrument Corporation,

Syosset, N.Y.

[22] Filed: April 12, 1971 [21] Appl.No.: 133,223

[52] US. Cl. ..350/201 [51] Int. Cl. ..G02b 17/08 [58] Field of Search ..350/20l, 199

[56] References Cited I Q UNITED STATES PATENTS 2,520,635 8/1950 Grey ..350/20l 3,547,525 l2/l970 Rayces et al ..350/20l X 4 1 Jan. 16,1973

Primary Examiner-David Schonberg Assistant Examiner--Paul A. Sacher Attorney-E. Manning Giles, J. Patrick Cagney, Michael A. Kondzella and Richard A. Zachar [57] ABSTRACT An objective system of high relative aperture for low-' 1 Claim, 6 Drawing Figures PATENTEDJAN 16 I975 SHEET 1 OF 2 FIG.|

FIG.2

INVENTORS MAX AMO/V ssmoun ROS/N BY- d 3 4 2 AT'TORNEY LARGE CATADIOPTRIC OBJECTIVE BACKGROUND OF THE INVENTION The present invention relates generally to catadioptrics and the more specifically to a large catadioptric objective arrangement.

SUMMARY OF THE INVENTION secondary spherical mirrors, the primary being a mangin mirror; large corrector elements forward of the primary, the rear surface of one of the elements being coincident with the secondary mirror; and field corrector means near the focal plane of the system.

The forwardmost of the corrector elements is positive so that the primary need not be larger than such element. All of the surfaces of the catadioptric can be spherical and a common glass can be employed throughout.

One objective lens system, in accordance with the invention and illustrated herein for specific use with an image intensifier incorporating an S-20 extended red sensor whose sensitive spectral region extends from 0.04 to 0.9 microns resolves 1,130 lines/mm on axis, and 500 lines/mm at the edge of the field, with considerations of observation and transmission, yielding a T/number of L93. The same objective system has been found to have a reasonable performance as far as 1.2 microns and, hence, anticipates the needs of future image intensifiers that are sensitive to spectral regions extending into the near IR.

Other features and advantages of the invention will be apparent from the following description and claims and are illustrated in the accompanying drawings which show structure embodying preferred features of the present invention and the principles thereof, and what is now considered to be the best mode in which to apply these principles.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings forming a part of the specification, and in which like numerals are employed to designate like parts throughout the same:

FIG. 1 is a schematic diagram illustrating a preferred embodiment of the objective lens system of the present invention;

FIG. 2 is a sectional view illustrating the housing and mountings for the objective lens system of FIG. 1;

FIGS. 3A-3D are performance charts for an objective lens system of the present invention.

DETAILED DESCRIPTION With reference now to the drawings and, specifically to FIG. 1, a large aperture objective system in accordance with the present invention comprises a catadioptric arrangement that includes a primary mirror 10, a secondary mirror 12, a number of relatively large corrector elements 14 forward of the primary l0, and field corrector means 16 near the focal plane P of the system.

In the illustrated embodiment, the corrector elements 14 comprise three large diameter lenses l8, l9 and 20 and the field corrector means 16 comprises a pair of smaller lenses 22 and 24. The first or forwardmost corrector element 18 is a positive lens therebyinsuring that the primary 10 need not be any larger than lens 18. The third or forwardleast of the correctors 20' is combined with the primary 10 to jointly therewith form a Mangin type mirror 26. In the preferred form the system is compensated so that the rear surface of thesecond-or intermediate lens 19 of the corrector elements I4 is made coincident with the secondary mirror 12. All of the surfaces of the objective lens system are spherical, a common glass being employed throughout. In the preferred form illustrated herein, F11 glass, developed by Schott, is employed because of its high index (1.620 at= 0.5876'microns), its efficiency with a single magnesium flouride anti-reflection coating (yielding 6 percent gain in transmission for the 10' airglass surfaces over the more common 1.5 index glasses), as well as because of its unusual properties of high rigidity and light weight.

FIG. 2 illustrates the housing and support structure for the objective. In FIG. 2 the two large corrector elements l8 and 19 are shown each supportedby and sealed into a main tubular support structure 32, the elements I8 and 19 being cushioned and centralized by potting compound around the circumference.

The field correctors 22 and 24 are sealed in a tubular cell 40 that can be axially adjusted to eliminate any residual spherical aberration.

In the illustrated embodiment position accuracy is basically built into the machining of the structural members by holding tolerances of 0.001 inches at most places, but it will be appreciated that enough versatility exists to use shimming if required. The air space between the Mangin 26 and the secondary mirror 12 is held to within i 0.006 inches and a more relaxed i 0.024-inch air space tolerance between the first and second elements l8, l9. Decentration of any of the lenses does not burden the system performance, and can be easily held within 10.01 inches to the optical axis.

The performance specifications for the illustrated embodiment are in terms of square wave transfer function (MTF). The minimum permissible transfer function for our requirements is 66 percent at 40 lines/mm for the axial case, and 38 percent modulation at 40 lines/mm at 2 off-axis. The computed and measured axial MTF of the objective is shown in FIG. 3. The axial MTF theoretically is 87 percent at 40 lines/mm, and it is anticipated that it could degrade as much as l0 per- 7 Radius (mm.) Thickness (mm.) Clear Surface (calculated) (center) Aperture (mm.)

a 969.92 15.0 (Glass) 232.0 6 116.70 (Air) 231.4 3 l 80.7 10.0 (Glass) 214.3 d 4778.6 133.13 (Air) 213.6 2 432.79 20.0 (Glass) 197.0 f v 631.40 20.0 (RefL) 199.2 g 432.79 133.13 (Air) h 4778.6 138.88 (Refl.) 110.4 1' 326.71 8.0 (Class) 64.2 j 492.95 7.47 (Air) 62.2 k 116.64 10.0 (Glass) 58.4 I 483.91 27.727 (Air) 55.4

Airspace adjustable 1 2.0 millimeters APER. STOP SURF b. 2.0

ENT. PUPIL DIA. 232.0 mm.

ENT.PUP1L LOC =9.315 mm.

EXT. PUPIL LOC 255.02 mm.

OVERALL LENGTH 332.78

EFL 345.8 mm.

BFL 26.727 mm.

f/No. 1.49 T.No. 2.0 SEMI FIELD ANGLE 3.2 SPEC. RANGE 0.4 to 0.9

While preferred embodiment is designed for S-20 extended red sensor whose sensitive spectral region extends from 0.4 to 0.9 microns, further analysis has shown that the system performance remains reasonable as far as 1.2 microns. This objective system of the present invention, therefore, anticipates the needs of future image intensifiers that are sensitive to spectral regions extending into the near IR.

While preferred constructional features of the invention are embodied in the structure illustrated herein, it is to be understood that changes and variations may be made by those skilled in the art without departing from the spirit and scope of the appended claims.

What is claimed is:

l. A catadioptric objective comprising a primary and a secondary mirror, three refractive correctors of relatively large clear aperture forward of the primary mirror and field corrector means comprising a pair of refractive elements of relatively small clear aperture near the focal plane, said objective being characterized v by the following parameters:

Radius (mm.) Thickness (mm.) Clear Surface (calculated) (center) Aperture (mm).

a 969.92 15.0 (Glass) 232.0 b 116.70 (Air) 231.4 c 3 1 80.7 10.0 (Glass) 214.3 d 4778.6 133.13 (Air) 213.6 e 432.79 20.0 (Glass) 197.0 f 432.79 20.0 (RefL) 199 2 g 432.79 l33.13 (Air) h 4778.6 138.88 (RefL) 110.4 1' 326.71 8.0 (Glass) 64.2 j 492.95 7.47 (Air) 62.2 k 116.64 10.0 (Glass) 58.4 1 483.91 27.727 (Air). 55.4

* Airspace adjustable 2.0 millimeters wherein surfaces a and b have reference respectively the forward and rear surfaces of the forwardmost of said correctors, surfaces 0 and d have reference respectively to the forward and rear surfaces of the intermediate of said correctors, surface c has reference to the forward surface of the forwardleast of said correctors, surface f has reference to the rear surface of the forwardleast corrector and to the surface of the primary mirror, surface g has reference to the surface of the secondary mirror, and surfaces h, i, j, k and l have A reference respectively to the surfaces of said pair of refractive elements. 

1. A catadioptric objective comprising a primary and a secondary mirror, three refractive correctors of relatively large clear aperture forward of the primary mirror and field corrector means comprising a pair of refractive elements of relatively small clear aperture near the focal plane, said objective being characterized by the following parameters: Radius (mm.)Thickness (mm.)ClearSurface(calculated)(center)Aperture (mm). a969.9215.0(Glass) 232.0b-116.70(Air) 231.4c-3180.710.0(Glass) 214.3d-4778.6133.13(Air) 213.6e- 432.7920.0(Glass) 197.0f432.79- 20.0(Refl.) 199.2g-432.79-133.13(Air) h4778.6138.88*(Refl.) 110.4i326.718.0(Glass) 64.2j492.957.47(Air) 62.2k116.6410.0(Glass) 58.4l483.9127.727(Air) 55.4* Airspace adjustable + OR - 2.0 millimeters wherein surfaces a and b have reference respectively the forward and rear surfaces of the forwardmost of said correctors, surfaces c and d have reference respectively to the forward and rear surfaces of the intermediate of said correctors, surface e has reference to the forward surface of the forwardleast of said correctors, surface f has reference to the rear surFace of the forwardleast corrector and to the surface of the primary mirror, surface g has reference to the surface of the secondary mirror, and surfaces h, i, j, k and l have reference respectively to the surfaces of said pair of refractive elements. 